1. Field of the Invention
The present invention relates to an image forming device, such as a color copying machine or a color printer, which processes an input image signal, thereby obtains an output image signal, and forms an output image on the basis of the output image signal.
2. Description of the Related Art
The remarkable improvement recently made on the picture quality of a color copying machine has given rise to a concern about forgeries of such specified documents as bank notes and securities.
In view of this situation, Japanese Patent Unexamined Publication No. Hei. 4-294682 proposes a process for writing in advance such information as an identification number unique to a given copying machine or a manufacturing number of the copying machine to a read only memory (ROM) built in the copying machine, for reading such information specifying the copying machine out of the ROM at the time of a copying operation, and for adding the specifying information in the form of a specific pattern which it is difficult for the human eyes to discern to the output images, so that it is made possible to identify the copying machine used for making a particular copy with reference to any copied object.
Further, Japanese Patent Unexamined Publication No. Hei. 4-332260 proposes a process for writing information in the form of a unique pattern in advance to a ROM built in a copying machine, for reading such a unique pattern of information out of the ROM at the time of a copying operation, and for synthesizing the unique identifying pattern in a color tone or density not readily discernible by the human eyes for its addition to the output images, so that it is indicated in the output images that the images are not any original but a copy.
Further, Japanese Patent Unexamined Publication No. Hei. 4-332260 cited above also proposes a method developed in consideration of a case in which a person who intending to falsify any particular document cuts off the electrical connection of the ROM or the pattern adding circuit mentioned above, removes the ROM or the pattern adding circuit, or replaces the ROM with a memory not containing any pattern information written to it. The proposed method prohibits any copying operation in case the system of the copying machine has recognized any disconnection or removal of the memory or the pattern adding circuit or any falsification of any pattern information by performing a check on the electrical connection of the ROM or the pattern adding circuit mentioned above in advance of a copying operation and thereby determining whether or not the memory or the pattern adding circuit is present in the copying machine and whether or not the information read out of the memory satisfy the prescribed conditions.